1. Field of the Invention
The present invention relates to an image forming apparatus and a method of controlling the same.
2. Description of the Related Art
An electrophotographic type color image forming apparatus generally includes a plurality of image forming units corresponding to different development colors. The respective image forming units form toner images of the respective development colors on photosensitive drums. In addition, the image forming apparatus forms a color image on a recording material held on a conveyance belt by sequentially superimposing and transferring the toner images of the respective colors, formed on the respective photosensitive drums, at the same position on the recording material. Alternatively, when the image forming apparatus includes an intermediate transfer belt, the apparatus forms a color image on the intermediate transfer belt by sequentially superimposing and transferring the toner images of the respective colors, formed on the respective photosensitive drums, at the same position on the intermediate transfer belt. Thereafter, the image forming apparatus forms a color image on the recording material by transferring the color image, formed on the intermediate transfer belt, onto the recording material (a case in which the image forming apparatus includes no intermediate transfer belt will be described below).
In such an image forming apparatus, the positions of toner images of the respective colors transferred from the photosensitive drums onto a recording material may differ from each other on the recording material, resulting in positional deviation (to be also referred to as “color deviation” hereinafter) due to the transfer of the respective toner images at positions different from each other. This color deviation can occur due to, for example, the unevenness of rotation of the respective photosensitive drums, the unevenness of movement of the conveyance belt, and fluctuations in the relative moving amounts between the outer surfaces of the photosensitive drums and the conveyance belt at the transfer positions of the respective image forming units for the respective colors. Particularly, in an image forming apparatus constituted by a plurality of image forming units each including a laser scanner and a photosensitive drum, if the distances between the laser scanners and the photosensitive drums in the respective image forming units differ from each other, the scanning widths of the lasers on the photosensitive drums fluctuate for the respective image forming units. This may cause color deviation.
As a technique for reducing such color deviation, there is known a technique of forming pattern images for color deviation detection, for the respective colors, on the conveyance belt which moves at a predetermined speed, and correcting color deviation based on the detection results obtained by detecting the respective pattern images using sensors provided on the downstream side. For example, speed fluctuations (unevenness of speed) of a long period equal to the rotation period of the conveyance belt occur in the conveying (moving) speed of the conveyance belt due to the unevenness of thickness of the conveyance belt. In addition, the moving speed of the peripheral surface of the conveyance belt may fluctuate due to rotating speed fluctuations occurring in the driving roller which drives the conveyance belt. In such a case, owing to the moving speed fluctuations of the peripheral surface of the conveyance belt, the transfer position to which a color deviation detection pattern image is transferred and the detection position where a pattern image is detected from the conveyance belt may change on the conveyance belt. This causes an error in the color deviation detection result obtained by using a color deviation detection pattern image. When such a color deviation detection error occurs, the accuracy of color deviation correction deteriorates, and the quality of an image formed on a recording material deteriorates.
To solve the above problems, for example, Japanese Patent Laid-Open Nos. 2001-356542 and 2005-316510 have proposed techniques. According to Japanese Patent Laid-Open No. 2001-356542, a plurality of detection pattern images are arranged at intervals corresponding to a fraction of an integer of the period on the conveyance belt in correspondence with speed fluctuation components in a specific period which cause color deviation detection errors. In addition, this technique reduces detection errors by averaging detection values obtained from the conveyance belt. Furthermore, the technique in Japanese Patent Laid-Open No. 2005-316510 reduces the influences of detection errors due to the moving speed fluctuations of the peripheral surface occurring at the same period as the rotation period of the conveyance belt, which can occur at the time of transfer of pattern images, by matching the transfer timings of color deviation detection pattern images with the transfer timings for all the colors.
The above conventional techniques, however, have the following problems. For example, according to Japanese Patent Laid-Open No. 2001-356542, when there are speed fluctuation components of a plurality of periods as speed fluctuation components whose influences should be removed, an attempt to reduce color deviation detection errors due to all the speed fluctuation components will increase the lengths of pattern images to be formed on the conveyance belt. This also increase the time required to detect the formed pattern images. In addition, in order to cope with speed fluctuations of a long period such as the period of the conveyance belt, it is necessary to form detection pattern images throughout the peripheral length of the conveyance belt, resulting in a large number of pattern images to be formed. This leads to an increase in the amount of toner consumed to form a detection pattern image.
The technique disclosed in Japanese Patent Laid-Open No. 2005-316510 gives consideration to detection errors occurring at the time of pattern image transfer as color deviation detection errors by the above speed fluctuations when using detection pattern images. On the other hand, the technique gives no consideration to detection errors occurring at the time of detection of pattern images. That is, according to the technique disclosed in Japanese Patent Laid-Open No. 2005-316510, matching the transfer timings of detection pattern images of all the colors with each other will match the magnitudes of the moving speed fluctuations of the peripheral surface of the conveyance belt at the transfer timings for all the colors. This can reduce color deviation detection errors. On the other hand, since different detection timings are set for detection pattern images of the respective colors, speed fluctuations of different magnitudes occur in the moving speed of the peripheral surface of the conveyance belt at the detection timings of the respective colors. As a result, detection errors due to fluctuations in the detection timings of detection pattern images for the respective colors remain in color deviation detection errors.